[giraffe_lady]

It's also been a while since I read it but he address some of those points in the extended series as they come up.

IIRC the two main things are that one of the main athenian sources greatly admired sparta and wanted athens to be more like it, so was actually biased in the other way than you'd expect. And in the other sources it's clear that some degree of respect for sparta's history and military tenacity was common among at least the athenian elite whose views we have access to.

Also in an extended conflict in a complex political environment involving other military powers, it can sometimes be to your benefit to acknowledge or even inflate the might of your enemy. A polity's regional stature is not improved by having a hard time defeating a weak opponent. So to some extent athenians probably contributed to sparta's enduring reputation as a formidable military power.

So yeah the sources are biased but mainly in ways that an informed-enough reader can recognize and account for and still draw useful information from them, albeit carefully. Which, I learned from this blog, is a big part of the actual practice of the discipline of history.